It seems to be that
a) Webhosts should keep their PHP versions to the latest stable version, mainly for security, but secondly for bug fixes and to take advantage of new features. PHP 5.2 came out in November last year and thus is PHP 5.2.0 - 5.2.3 would be my current description for up to date.
b) The only reason for keeping back is that scripts known to work on 5.0/5.1 may not work with 5.2, and there's time involved in testing this (we know this full well, we have a server in this state).
c) Therefore we should encourage (but not require) those installing SilverStripe to use PHP 5.2, so that SilverStripe runs as securely and bug-free as possible, and as a bonus, allows us to make use of the (minor as I see it) new features.
I see this done as a notice in the installer saying phrases like "we encourage using 5.2 for SilverStripe... your version of PHP is old, buggy and insecure ... your current version will work now, however later versions of SilverStripe may not work unless you upgrade to 5.2"
d) With the view that by [url=http://www.gophp5.org/]February 5, 2008[/url], SilverStripe will altogether officially drop support for PHP 5.0/5.1, which specifically means not testing it on those versions, and using code that is known to break on those versions.
e) Its important to realise that SilverStripe's minimum version is slowly increasing anyway, as we find major use of functions or code that breaks due to PHP bugs. For instance, PHP 5.0.3 had a number of bugs where we decided not to deal with, and instead say "upgrade" to. So, I feel we're essentially doing (e) without realising it, but it is healthier for us to definitively act on this, and encourage use of installing SilverStripe of PHP 5.2 webhosts so that we can make use of it.
f) As a side note, one major reason large packages (anything from SilverStripe1 or Drupal) get themselves into trouble is through holding onto an old version. By incrementally lifting the minimum PHP version, it will be easier to eventually make use of PHP 5.3 or 6.0 features, so this is all part of a long term strategy of keeping SilverStripe in the lead.
See PHP 5 Changelog: [url]http://www.php.net/ChangeLog-5.php[/url]